The present invention relates to phase control devices and more particularly to a received signal phase control device employed in a modulation circuit for a compatible quadrature AM stereophonic signal which is compatible with both stereophonic and monophonic receivers.
An AM stereophonic signal produced with a compatible quadrature phase modulation (PM) system is known as a stereophonic signal. Such a stereophonic signal e.sub.i can be, in general, expressed by the following equation (1): EQU e.sub.i =[{1+k[L(t)+R(t)]}.multidot.cos .omega.it+k[L(t)-R(t)].multidot.cos (.omega.it+.pi./2)].multidot.cos .phi.. (1)
where .phi.=tan.sup.-1 k[L(t)-R(t)]/{1+k[L(t)+R(t)]}, L(t) and R(t) are the left and right channels, respectively, .omega.i is the angular frequency of the carrier signal, and k is the modulation factor.
That is, the compatible quadrature AM stereophonic signal expressed by the equation (1) is a combination of a signal which is produced by modulating the amplitude of the carrier signal, cos .omega.it, with a signal corresponding to the sum of the two channel signals and a signal which is produced by modulating the amplitude of the carrier signal phase shifted by 90.degree., cos (.omega.it+.pi./2), with a signal corresponding to the difference between the two channel signals. In order for the stereophonic signal to be usable by a monophonic receiver, signal transmission is effected by modulating the level of the composite signal with cos .phi. as indicated in the equation (1).
The equation (1) can be rewritten as the following equation (2): EQU e.sub.i =[{1+kL(t)}.multidot.cos (.omega.it+.pi./4)+{1+kR(t)}.multidot.cos (.omega.it-.pi./4)].multidot.cos .phi./.sqroot.2. (2)
An example of a system for demodulating the stereophonic signal indicated by the equation (2) is shown in FIG. 1. In this system, a received input signal is converted into an intermediate frequency signal e.sub.i which is amplified by an IF (intermediate frequency) amplifier 3 and is then applied to a divider 4. In the divider 4, the cos .phi. component of the intermediate frequency e.sub.i is removed and the resultant signal is applied to a quadrature stereophonic demodulator circuit 5. The quadrature stereophonic demodulation circuit 5 is composed of a product demodulator in the form of a differential circuit. The quadrature stereophonic demodulation circuit 5 provides as outputs the product of the output of the divider 4 and a signal component cos (.omega.it+.pi./4) and the product of the output of the divider 4 and a signal component cos (.omega.it-.pi./4), which outputs are the signal components L(t) and R(t), respectively. In order to provide the signal components cos .phi., cos (.omega.it+.pi./4) and cos (.omega.it-.pi./4), a phase-locked loop (PLL) circuit 10 and phase shifters are employed.
The intermediate frequency signal e.sub.i, after being converted into a square wave by a limiter 6, is applied to an input terminal of a phase comparator 7. The output of the phase comparator 7 is applied through a low-pass filter (LPF) 8 to a DC amplifier 9 where it is amplified and is then applied, as a control voltage, to a voltage-controlled oscillator (VCO) 11. The output e.sub.o of the voltage-controlled oscillator 11 is applied to the other input terminal of the phase comparator 7. As a result, the phase comparator 7 produces an error voltage V.sub.1 corresponding to the frequency and phase difference of the input signal e.sub.i applied to the first input terminal of the phase comparator 7 with respect to e.sub.o.
The output e.sub.o of the voltage-controlled oscillator 11 is applied to a .pi./2 phase shifter 12 where it is shifted by 90.degree. in phase and is then applied to a first input terminal of an in-phase detector 13. The intermediate frequency signal e.sub.i is applied to the other input terminal of the in-phase detector 13 as a result of which the detector 13 produces as an output the cos .phi. component which is applied to the divider 4.
The output of the .pi./2 phase shifter 12 is further applied to a .pi./4 phase shifter 14 and a .pi./4 phase shifter 15 where the phase of the output of the phase shifter 12 is shifted by +45.degree. to -45.degree. to produce the components cos (.omega.it+.pi./4) and cos (.omega.it-.pi./4), respectively. These components are applied to the quadrature demodulation circuit 5.
With the phase comparator 7 in the PLL circuit 10 producing an output voltage signal V.sub.1 which is proportional to the cosine of the phase difference between the two input signals, the phase difference .DELTA..phi.e of the two input signals is: EQU .DELTA..phi.e=cos.sup.-1 .DELTA..omega./Kd, (3)
where Kd is the loop gain of the PLL circuit and .DELTA..omega. is the difference between the angular frequency .omega.i of the input signal e.sub.i and the free-running angular frequency of the voltage-controlled oscillator 11.
Thus, it is clear from the equation (3) that when .DELTA..omega. is zero, that is, when the input signal e.sub.i is equal to the free-running frequency of the voltage-controlled oscillator, .DELTA..phi.e is 90.degree. and the phase of the output e.sub.o of the voltage-controlled oscillator 11 is shifted by 90.degree. from the phase of the input signal e.sub.i. Thus, the signal components cos .phi., cos (.omega.it+.pi./4) and cos (.omega.it-.pi./4) obtained with the use of the signal e.sub.o have the desired phase relationships thereby permitting accurate quadrature stereophonic demodulation to be carried out.
However, if the frequency of the local oscillator signal e.sub.L is somewhat shifted in frequency due to temperature change or the like, then the frequency of the intermediate frequency signal e.sub.i will also be shifted as a result of which the difference .DELTA..omega. will not be zero. In this case, the phase difference .DELTA..phi.e between the signal e.sub.i and the output signal e.sub.o of the voltage-controlled oscillator varies with the angular frequency difference .DELTA..omega. as indicated by the solid line 201 in FIG. 2.
As is apparent from the above description, it is known in the art that when the free-running frequency of the voltage-controlled oscillator is different from the frequency of the input signal e.sub.i, the output e.sub.o of the voltage-controlled oscillator 11 is locked and follows the input signal although the phase thereof has a predetermined amount of shift .DELTA..phi.e from the input signal e.sub.i. Therefore, although the output of the voltage-controlled oscillator should be the signal e.sub.o whose phase is shifted exactly by 90.degree. from that of the input signal e.sub.i, the actual output is shifted by 90.degree..+-..alpha. from that of the input signal so that correct quadrature demodulation is not carried out.